There are recent polls of Sanders leading Trump in Michigan, Pennsylvania, Florida, North Carolina, and Wisconsin. There's even one from October where he's leading in Ohio.
Of course, it's early. And, of course, polls will fluctuate. And there is a lot of volatile stuff going on with the virus and the economy that can change things. Plus there will undoubtedly be some other surprises.
But what I don't understand is why so many people think these polls are so overwhelming likely to swing in Trump's favor.